nextnano - Software for
semiconductor nanodevices

We apply the Contact Block Reduction (CBR) method to a simple GaAs nanowire of cuboidal shape. The corresponding tutorial for nextnano³ is here.

We consider a GaAs cuboidal tube of dimensions 10 nm$\times$10 nm$\times$20 nm. Two leads of 10 nm$\times$10 nm each are attached to the edge of the device. The grid spacing is 1 nm in all directions. The effective electron mass is assumed to be constant throughout the device and equal to $0.067m_0$.

To simulate 3D (or 2D) system with CBR method in nextnano++ correctly, The quantum regions have to be appropriately specified in the input file.

quantum{
	region{
		name = "lead_1"
		x    = [-6,6]
		y    = [-6,6]
		z    = [-0.1,0.1]
		boundary{ x=dirichlet y=dirichlet z=cbr }
                Gamma{ num_ev = $num_eigenstates_device }
	}
}

The perpendicular directions, i.e. x- and y-directions, of the system are elongated by one grid due to the treatment of edge points in nextano++. Since the simulation is three dimensional, the lead region specified here has to be two dimensional. The number $\pm0.1$ is chosen to be smaller than the grid spacing, so that the region “lead_1” becomes a 2D sheet (Note: this is slightly different in nextnano³ input). CBR boundary condition has to be imposed in the propagation direction, i.e. z-direction, whereas Dirichlet boundary condition is set for perpendicular directions.

cbr{
         name = "device"
         lead{ name = "lead_1" }
         lead{ name = "lead_2" }
         delta_energy   = $delta_energy
         abs_min_energy = $E_min
         abs_max_energy = $E_max
         options        = [1, 0, 0]
}

Here we specify the device region and leads attached to the device. The program calculates transmission through the region “device”, from “lead_1” to “lead_2”. The resolution, minimum and maximum of the energy axis can be also tuned here.

<figure transmission> <caption>Transmission coefficient of a GaAs 3D nanowire. Arrows indicate the cut-off energies, namely the eigenenergy of the highest device eigenmode considered in each simulation.</caption> </figure>